The present invention relates to optical display systems, and in particular, an optical display system that comprises a holographic optical element which reflects light image information toward an observer so that he can view the information from anywhere within a head motion volume and not perceive variations in image brightness.
The problems with prior optical display systems relating to image brightness uniformity and display contrast ratio are exemplified by those experienced with head-up display systems for aircraft.
The design of conventional head-up display systems that employ spectrally insensitive dielectric combiners entails a trade-off between real-world photopic transmission through and cathode ray tube energy reflection by the combiner. The photopic transmission and energy reflection characteristics of a combiner affect the contrast ratio of the display for a given image source light output and an observer's ability to view image information against an ambient background of high brightness. A typical spectrally insensitive dielectric combiner has a 70% maximum transmissivity and a 20% maximum reflectivity. The 20% maximum reflectivity indicates that 80% of the light energy emanating from the cathode ray tube never reaches the observer. To produce a light output of sufficient intensity for an acceptable contrast ratio against a background of high brightness, the cathode ray tube must be driven at a high beam current, which has the deleterious effect of shortening its usable life. Combiners constructed of spectrally insensitive dielectric materials, therefore, do not promote good contrast in a head-up display system of efficient design.
Holographic combiners have heretofore been used in head-up display systems to ameliorate the trade-off between real-world scene photopic transmission and cathode ray tube energy reflection. The reason is that wavelength selectivity is a natural attribute of a hologram.
Although a hologram can reflect or diffract a narrow band of wavelengths with high efficiency, there exists a duality between the spectral bandwidth and angular bandwidth of a hologram, which duality creates difficulties in achieving image brightness uniformity over an entire field of view or with pilot head motion. A hologram at a fixed angle has a finite spectral bandwidth because during playback its diffraction efficiency diminishes rapidly as the wavelength of cathode ray tube energy varies from the peak efficiency wavelength, i.e., the Bragg wavelength condition. Similarly, the hologram has a finite angular bandwidth because it does not reflect light rays of the cathode ray tube wavelength at angles outside of a narrow range centered about the Bragg angle. The peak efficiency wavelength and angle are established during the hologram exposure and processing.
Providing a display image of perceptibly uniform brightness requires, therefore, that the hologram reflection characteristic be tailored to present such an image to the observer, irrespective of the position of his head within a specific head motion volume. The head motion volume is defined as the region in space through which the observer can move his head and see a display image. Achieving brightness uniformity is a problem because of the combined effects of the hologram reflection and cathode ray tube phosphor emission characteristics at the head-center position of the head motion volume. Since the hologram has angular bandwidth, the reflection efficiency diminishes as the observer moves his head in the vertical direction from the head-center position. The reflection efficiency near the vertical limits of the head motion volume is generally insufficient to promote acceptable brightness uniformity of the display image.
One proposed solution to the problem of brightness uniformity is the construction of a hologram of sufficient angular bandwidth that covers the expected range of vertical head motion. The disadvantages of the conventional wide band hologram approach are that there is rejection of a wide band of wavelengths from the real-world scene as viewed from the combiner, which results in poor photopic transmission; excessive real-world coloration or tinting; and perceptible brightness variations. The use of a wide band hologram introduces, therefore, a trade-off among photopic transmission, real-world coloration, and brightness uniformity.
A wide band hologram designed with a nonsaturated peak diffraction efficiency results in a corresponding increase in spectral bandwidth that improves the brightness uniformity across the field of view. There exist, however, several problems with the nonsaturated hologram. First, the integrated efficiencies of the hologram reflection and cathode ray tube phosphor emission characteristics are so low that the cathode ray tube must be driven at a relatively high beam current to maintain an acceptable contrast ratio against backgrounds of high brightness. (Integrated efficiency is defined herein to mean the area under the curve which is the multiplication product of the hologram reflection characteristic and the cathode ray tube phosphor emission characteristic for a given head position and look angle.) Driving the cathode ray tube in this manner decreases its usable life. Second, a nonsaturated hologram is typically constructed in a relatively thin recording material. The sensitivity of the integrated efficiency of the thin hologram imposes strict thickness control requirements for good brightness uniformity. Third, variations in the transmissivity of the hologram result from variations in its integrated efficiencies.
The design of head-up display systems that employ spaced-apart, partly overlapping lower and upper reflective coatings involves an additional consideration as respects the use of wide band holograms to improve image brightness uniformity. The lower combiner must reflect simultaneously the image information emanating from the cathode ray tube at angles larger than the critical Bragg cutoff angle and transmit the same image information at angles smaller than the critical Bragg cutoff angle. Increasing the spectral bandwidth of the lower combiner improves brightness uniformity but does not permit the efficient transmission of image information through the lower combiner to the upper combiner.